The objective of this research is to investigate the cellular and tissue mechanisms responsible for bronchiolar hypoplasia and alveolar dysplasia that result from viral bronchiolitis and pneumonia during early life. Rats infected with parainfluenza type 1 (Sendai) virus are the animal model used. Morphometric and autoradiographic methods are being used to investigate changes in cell proliferation and cell numbers that occur during postnatal lung growth and that are altered by viral-induced tissue damage and inflammation. In addition, the long-term effects of viral bronchiolitis on pulmonary function and airway hyperreactivity in rats are being assessed. Inbred strains of rats that vary in their susceptibility to viral bronchiolitis will be identified for future studies aimed at investigating genetic and immunological mechanisms that are important in controlling the development of pathologic sequelae to viral bronchiolitis in infancy.